The purpose of this study was to assess whether understanding relational terminology (i.e., more, less, and fewer) mediates the effects of intervention on difference word problems. Second-grade teachers who volunteered to participate were assigned to 1 of 3 conditions: schema-broadening word-problem intervention, calculation intervention, or business-as-usual control. Students within the word-problem intervention condition received explicit instruction on the difference problem type, which included a focus on understanding relational terminology within word problems. Analyses, which accounted for the nested structure of the data, indicated that, compared to the active and inactive contrast conditions, word-problem intervention significantly increased performance on difference problems and on understanding relational terminology and that those intervention effects on difference problems were partially mediated by students’ understanding of relational terminology.